Orthopedic implant

ABSTRACT

Disclosed is an orthopedic implant suitable for arthroplasty procedures. The orthopedic implant includes a first plate, a second plate, an axial support between the first plate and the second plate and one or more torsional supports connecting the first plate and the second plate. The axial support may be, for example, one or more flexible struts, such as cables, or a ball and socket joint. The torsional supports connect the first and second plates and may be, for example, curved around the axial support. The torsional supports may be integrally formed with the first and second plates as a single unitary device, by, for example, a Laser Engineered Net Shape (LENS) process.

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 60/137,626, filed Jun. 4, 1999, titledMETHOD, APPARATUS, DESIGN AND MANUFACTURE OF DEVICES FOR TISSUE REPAIR,TRANSPLANTATION AND SURGICAL RECONSTRUCTION.

FIELD OF THE INVENTION

The present invention is directed to an orthopedic implant. Morespecifically, the orthopedic implant is suitable for arthroplastyprocedures where optimized multifunctional behavior of the implant isdesired. The implant may include the ability for load sharing betweenthe implant and host bone, and the restoration of motion, such as in thereplacement of a spinal disc.

DESCRIPTION OF THE RELATED ART

Orthopedic implants have been used in the past to repair damage to theskeleton and related structures, and to restore mobility and function.For example, various devices, such as pins, rods, surgical mesh andscrews, have been used to join fractured bones in the proper orientationfor repair.

Implants that restore the function to a damaged joint have also beenused. Surgery intended to restore function to a joint is referred to asarthroplasty. A successful arthroplasty may eliminate pain and preventthe degradation of adjacent tissue. Arthroplasty has been performed onknees, hips and shoulders by replacing portions of the joint withimplants. Presently available implants for arthroplasty may result instress shielding, meaning that the stress normally felt by bone adjacentto the implant is reduced due to the stiffness of the implant. When abone is stress shielded, it typically reduces in size and strengthaccording to Wolf's Law, increasing the chance of breakage.

In some instances, instead of replacing a damaged joint, the joint ismerely fused in a single position. Surgery intended to fuse a jointrather than to restore mobility is referred to as arthrodesis.Arthrodesis is particularly common for the complex load-bearing jointsof the spine. Spinal fusion may be performed to remedy failure of aspinal disc. Spinal discs perform spacing, articulation, and cushioningfunctions between the vertebrae on either side of the disc. If thenormal properties of a disc are compromised, these functions can beseriously reduced. Disc collapse or narrowing reduces the space betweenvertebrae, and damage to the disc can cause it to bulge or rupture,possibly extruding into the spinal canal or neural foramen. Thesechanges can cause debilitating back and distal pain, numbness, orweakness.

Orthopedic implants may be used in arthrodesis to stabilize the spineand promote fusion. The two main approaches to implant-aided spinalfusion are anterior and posterior. Anterior fusion techniques are widelyused, primarily due to the recent appearance of Interbody Fusion Devices(IBFDs). IBFDs are inserted from an anterior approach into the spacenormally occupied by the disc for space retention, stabilization andload bearing. Posterior fusion is accomplished by cutting through themusculature of the back, exposing the spinal segments, and fixingadjacent vertebra using hardware typically consisting of metal rods,screws, and other devices. Bone harvested from the patient's iliac crest(autograft), donor bone (allograft), or other synthetic biocompatiblematerial is sometimes also packed into the space to induce fusion.

U.S. Pat. No. 5,860,973 (Michelson) discloses an implant which is placedtranslaterally between two discs. The implant, which is typicallyinstalled as a pair of implants, is cylindrical and is filled withfusion promoting material. During the installation, holes are boredbetween the vertebra and the implant is placed within the holes. Thevertebra then grow toward one another and fuse together.

Another way to treat spinal damage is to replace the damaged vertebra ordisc with some form of spacer. For example U.S. Pat. No. 5,702,451(Biedermann) discloses a space holder for a vertebra or spinal discconsisting of a hollow sleeve perforated with diamond-shaped holes. Theholes are sized and arranged such that when different lengths of sleeveare cut, the recesses along the edge of the cut resulting from thediamond shaped holes are uniform and able to be mated with projectionson an end cap.

Both spinal fusion, such as disclosed by Michelson, and the use ofspacers, such as disclosed by Biedermann, limit the mobility of thespine by fixing two adjacent vertebra relative to one another. Inaddition to reduced mobility, these arrangements do not compensate forthe shock absorption lost when a disc is damaged or removed.

Attempts to restore lost function to damaged spinal joints(arthroplasty) have also been made. For example, replacement of entirediscs or simply the nucleus pulposis (center portion of the disc) havebeen proposed. Some attempts use elastomers to mimic the shockabsorption and flexibility of the natural disc. However, the complexload bearing behavior of a disc has not been successfully reproducedwith an elastomer, and such implants are prone to wear and failure. Forexample, U.S. Pat. No. 5,674,294 (Bainville) describes an intervertebraldisc spacer having two metal half-envelopes which confine between them acushion. Similarly, implants using various liquids and gels have alsobeen attempted. These implants are subject to failure by rupture ordrying out, just like a disc. Mechanical approaches to disc replacementhave also been attempted. For example, articulating surfaces andspring-based structures have been proposed. In addition to failing toaccurately perform the functions of the replaced disc, these structuresare multi-component and particles due to wear of articulating componentscan result in adverse biological responses or increase the possibilityof mechanical failure. For example, U.S. Pat. No. 5,893,889 (Harrington)describes an artificial disc having upper and lower members joined by apivot ball and having a shock absorbing members fitted between the upperand lower member.

Total hip arthroplastics that use rigid stems as the load sharingdevices between the femur and the acetabulum have been observed toexperience the phenomena referred to as stress shielding also. In thiscase, the method of load transfer has been changed with the insertion ofthe implant. In the normal femur, the loads are applied to the femoralhead and transferred along the length of the femur through the corticalshell of the femur. In the case of the femur with an implant, the loadsare applied to the prosthesis which transfers the loads distally downthe prosthesis and gradually transfer the loads from the prosthesis tothe inside of the cortical shell. This results in a significant portionof the proximal portion of the femur no longer experiencing a normalstress condition. This condition will then result in a loss of bone masssurrounding the distal portion of the device. Consequences of this boneloss include loss of support for the device, which will allow the deviceto move and become painful, and, should revision of the device required,then there may be insufficient bone for support of the subsequentimplant.

A number of approaches have been attempted to solve this problem. Theseinclude use of composite materials for controlled stiffness of the bulkmaterial, modifications of the cross section of the device to reducestiffness (this includes local reduction in cross section and hollowstems) and incorporation of slits in the device to increase flexibility.None of these approaches have been successful in that the compromisesrequired to achieve the reduction in stiffness did not find the propercompromise between the required strengths and stiffness.

In total knee arthroplastics wear surfaces are typically made up of twomaterials, a polymer and a metal. Typically, ultra high molecular weightpolyethylene (UHMWPE) is used as the polymer. While this material hasexcellent wear properties, it is not a wear free surface. The cartilageof the normal knee is capable of producing a fluid film upon theapplication of mechanical stresses to it. This fluid film is then usedas a lubricant to reduce the coefficient of friction between the twocartilage wear surfaces. There is no fluid film lubricant in the totalknee joint implants presently known. Instead the materials articulatedirectly on one other resulting in the generation of wear debris andpossibly adverse biological responses.

Several attempts have been made to incorporate stochastic foam materialsto reproduce this fluid film lubrication mechanism in the knee joint.However none of these approaches have been successful in reproducing thefunctionally graded material properties required for this application.

SUMMARY

Accordingly, one embodiment of the present invention is directed to anorthopedic implant including a first plate, a second plate, an axialsupport between the first plate and the second plate, and one or moretorsional supports connecting the first plate and the second plate. Thetorsional supports, the first plate and the second plate may beintegrally formed.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred, non-limiting embodiments of the present invention will bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 is a perspective view of one embodiment of an orthopedic implantof the present invention;

FIG. 2 is a perspective view of another embodiment of an orthopedicimplant of the present invention;

FIG. 3 is a perspective view of another embodiment of an orthopedicimplant of the present invention; and

FIG. 4 is a perspective view of another embodiment of an orthopedicimplant of the present invention.

DETAILED DESCRIPTION

The design of an optimized implant for use in arthroplasty, such asspinal disc replacement, may be achieved through several methods.Preferably, such a device is designed using functionally adaptedsoftware, such as that described in U.S. patent application Ser. No.09/400,516, titled “METHOD AND APPARATUS FOR DESIGNING FUNCTIONALLYADAPTED STRUCTURES HAVING PRESCRIBED PROPERTIES,” which is hereinincorporated by reference. This design methodology involves the use of aseed geometry that has been screened for a prescribed set of mechanicalproperties. The seed geometry may be adjusted to fill a design envelopedefined as the space available for an implant as determined fromanatomical studies. A set of inputs may then be determined representingthe functional requirements for a clinically successful implant. Thenext step may be to optimize the seed geometry using functionallyadapted software, such as that described in the above-mentioned U.S.Patent Application. In the case of a lumbar spinal disc replacement,loading conditions and corresponding stiffness requirements may be usedas the input conditions for the optimization algorithm. Additionalcriteria also may be applied, such as maintaining peak stresses at astress level at or below the fatigue endurance limit of the material theimplant is to be fabricated from.

In addition to functionally adapted software mentioned above, there areseveral other approaches that can be taken to optimize the structure.For example, commercially available software packages such as ProEngineer produced by Parametric Technologies Corporation of Waltham,Mass. and ANSYS produced by ANSYS, Inc. of Canonsburg, Pa. may be usedto optimize some parameters based on, for example, geometricconsiderations, material properties and the functional properties of thejoint to be replaced.

The seed geometries used for the optimization process may be taken froma library of three dimensional geometries such as that available fromMolecular Geodesics, Inc. of Boston, Mass. Examples of these geometriesare structures such as octet trusses and kelvin foams. These seedgeometries may be combined in a continuous or discontinuous manner. Thecombination of these geometries is known as combinatorial geodesics.

The seed geometries need not be homogeneous. Instead, for example, ifanisotropic properties are desired, the seed geometries may be adjustedsuch that these are also anisotropic. Thus, an implant may includediffering seed geometries. These seed geometries may be solid, porous orother standard manufacturing constructs such as braids, woven materialsor laminates. Furthermore, the seed geometries are not required to beconstant in cross section, instead, the geometric properties of thecross section may be varied throughout the structure.

One key feature to the design of implants using the techniques describedabove is the recognition that designs do not have to be limited to thetraditional manufacturing constraints such as those imposed byconventional machining or casting methods. These methods havelimitations regarding the size and shape of the features that may beproduced. Construction of implants designed using the techniquesdescribed above may be with the use of other manufacturing techniquessuch as solid free form fabrication. Some examples of solid free formfabrication include, but are not limited to, directed deposition ofmetals (also known as Laser Engineered Net Shape [LENS] processes),Selective Laser Sintering (SLS) and 3D printing. All of these approachesmay be used in combination with the Hot Isostatic Pressing (HIP) processto produce a product substantially free of internal porosity anddefects. The LENS process includes directing a stream of metal powderinto a mobile laser which melts the metal. As the laser moves, the metalsolidifies. Subsequent layers of metal may be deposited on one another,allowing a three dimensional structure to be built up. The LENS processis described more fully in U.S. Pat. No. 6,046,426 (Jeanette) and U.S.Pat. No. 5,993,554 (Keicher) and these patents are hereby incorporatedby reference. The combination of these manufacturing techniques and theoptimization approaches described above allow for the design,optimization and manufacture of novel structures with multifunctionalfeatures according to the invention. In particular, according to theinvention, there are provided novel implant structures, such as unitarystructures, that have significant variations in stiffness in the axialand flexion/extension orientations.

Referring now to the figures, and in particular to FIG. 1, one exampleembodiment of an orthopedic implant according to an embodiment of theinvention is illustrated. Orthopedic implant 10 includes a first plate100, a second plate 102, an axial support 200 between plates 100, 102and one or more torsional supports 300A, 300B connecting the first plateand the second plate. As used herein, the axial means along an axis thatis substantially perpendicular to the primary surfaces of plates 100,102 and an axial support is a structure that provides support andresistance to compression in the axial direction. As used herein,torsion refers to both twisting and bending and torsional support refersto a structure providing resistance and support against twisting orbending.

First and second plates 100, 102 may be of any material and constructedin any manner that allows plates 100, 102 to establish a stableinterface with adjacent tissue and that are safe for an implantrecipient. For example, where implant 10 completely replaces a jointbetween two bones, plates 100, 102 may be constructed to establish astable interface with adjacent bone. Establishing a stable interface mayhave both short and long term components. Specific structure may beincluded on plates 100, 102 to address each component. For example,plates 100, 102 may have structure ensuring that implant 10 remains in adesired location in the short term, following implantation. Thisstructure may include, for example, protrusions 400, as illustrated inFIG. 4, such as, for example, teeth, ridges or serrations. Plates 100,102 may also be constructed to interact with other fixation devices. Forexample, plates 100, 102 may have holes 12 for receiving bone screwswhich may be used to affix them to the bone. Similarly, plates 100, 102may have structure ensuring that implant 10 remains in a desiredlocation in the long term, and successfully interfaces with adjacenttissue. This structure may include, for example, a tissue ingrowthregion 500 (See FIG. 4) that allows adjacent tissue to grow into theimplant, forming a stable interface. Tissue ingrowth region 500 mayinclude porous surfaces, osteoinductive surfaces and osteoconductivesurfaces. For example, tissue ingrowth region 500 may comprise sinteredmetal particles or a structure constructed by solid free formfabrication. The tissue ingrowth region 500 may only include enough ofplates 100, 102 to establish a stable interface with adjacent tissue andplates 100, 102 may be predominantly solid.

It is to be appreciated that first and second plates 100, 102 may alsobe sized and shaped to establish a stable interface with adjacenttissue. For example, plates 100, 102 need not be flat and may be shapedto match the contour of adjacent tissue. For example, if the tissueadjacent to one of plates 100, 102 is concave or convex, the plate 100,102 may be constructed with a curved shape to match the adjacent tissueof the joint. Similarly, plates 100, 102 need not be circular or oval asillustrated in FIGS. 1-4, rather, they may be any shape that allowsestablishment of a stable interface with adjacent tissue. Accordingly,implant 10 may have an irregular shape corresponding to an adjacenttissue such as a bone. For example, where implant 10 is used to replacean intervertebral disc, it may be shaped like a spinal disc, allowing itto fit easily between the vertebra and to establish a stable interfacetherewith.

It is to be appreciated that first and second plates 100, 102 may beconstructed of any material that is safe for a recipient of implant 10,and that allows a stable interface with adjacent tissue. For example,plates 100, 102 may be constructed of a material that is biocompatible,meaning that it is neither harmful to the health of an implantrecipient, nor significantly damaged or degraded by the recipient'snormal biology. Biocompatible materials include, for example, variousmetals and metal alloys, ceramic materials and synthetic materials, suchas polymers. It is also to be appreciated that plates 100 may also beconstructed of a material that is strong and durable enough to withstandthe forces that may be placed upon it once installed in an implantrecipient. For example, if implant 10 is used to replace a load bearingjoint, the material for plates 100, 102 may be selected such that plates100, 102 will not fail under stresses normally experienced by thatjoint, The ability of plates 100, 102 to withstand stresses also may bedependant on the shape and size of plates 100, 102 as well as theirmaterial of construction and, thus, it is to appreciate that the sizeand shape of plates 100, 102 may also be considered when selecting amaterial. In one embodiment of an implant according to the invention,plates 100, 102 are preferably constructed of titanium or a titaniumalloy. A titanium alloy typically used in implants includes 6% aluminumand 4% vanadium by weight.

Referring now to FIGS. 1-3, it is to be appreciated that axial support200 may by constructed of any materials and in any manner that providessufficient support and flexibility for a successful arthroplasty and issafe for the implant recipient. Axial support 200 may be constructed ina manner so that it provides support along an axis that is substantiallyperpendicular to the primary surfaces of plates 100, 102. This supportmay be sufficient for implant 10 to successfully bear loads that may beplaced upon it, However, axial support 200 may also be constructed in amanner so that it provides flexibility so that it may successfullyrestore motion to a joint it replaces. For example, axial support 200may be constructed as one or more struts or of first and second matinghalves 202, 204 that provide sufficient support and flexibility toimplant 10.

Where axial support 200 is constructed as one or more struts, the strutsmay be constructed to provide axial support to implant 10 and also to beflexible. For example, the struts may be relatively incompressible andmay be arranged substantially perpendicular to plates 100, 102 asillustrated in FIG. 1. Accordingly, stress applied parallel to, anddirectly above, the struts will be resisted by each strut due to itsincompressibility. Conversely, stresses not parallel to, or directlyabove, the struts will result in bending of the struts due to theirflexibility.

In one embodiment of the implant of the invention, the struts may becables. Cables are typically relatively incompressible along theirlengths and are also typically flexible. Accordingly, an axial support200 constructed of one or more cables would provide support sufficientto replace the load bearing function of a joint while also allowing itto flex, resulting in a successful arthroplasty implant.

Axial support 200 may also comprise a single strut shaped to beflexible. For example, axial support 200 may comprise a strut that istapered in a center region, as illustrated in FIGS. 2 and 4. Althoughthe strut is illustrated in FIGS. 2 and 4 as having an ovalcross-section, any shape providing the desired support and flexibilityfor axial support 200 may be used.

Where axial support 200 is constructed of mating halves 202, 204, it maybe constructed to provide sufficient support and flexibility for asuccessful arthroplasty and to provide a stable connection betweenhalves 202, 204. Halves 202, 204 constructed to provide a stableconnection may be constructed such that they may not slip off of oneanother or otherwise become detached from one another. Halves 202, 204may also be constructed such that axial support 200 is still flexible.For example, halves 202, 204 may form a joint capable of articulation,such as, for example, a ball and socket joint that may be sufficientlystable to withstand stresses typically applied to a joint, yet that willnot detach, and that is articulable to provide flexibility to axialsupport 200.

It is to be appreciated that axial support 200 may be located anywherebetween plates 100, 102 to provide support and flexibility at anyportion of plates 100, 102. The location of axial support 200 may dependon the nature of implant 10 and the type of joint being replaced.Typically, axial support 200 will be located at the point about whichthe joint it replaces normally pivots. This may be near the center ofplates 100, 102, however, it need not be. Axial support 200 may beconnected to the plates 100, 102 by any method that will maintain it ina proper location and is not subject to failure. For example, axialsupport 200 may be welded to plates 100, 102, or it may be integrallyformed with plates 100, 102 such as described above using the LENSprocess.

It is to be appreciated that axial support 200 may be constructed of anymaterial that is safe for a recipient of implant 10 and can withstandthe stresses and friction that will be placed upon it. For example,axial support 200 may be constructed of a material that isbiocompatible. Axial support 200 may also be constructed of a materialthat is strong and durable enough to withstand the forces that may beplaced upon it once installed in an implant recipient. For example, ifimplant 10 is used to replace a load bearing joint, the material axialsupport 200 is constructed from may be selected such that axial support200 does not fail under stresses normally experienced by that joint. Theability of axial support 200 to withstand stresses also may be dependanton the shape and size of axial support 200 as well as its material ofconstruction and, thus, size and shape of axial support 200 may also beconsidered when selecting a material. Where axial support 200 alsocomprises an articulating joint, the material that axial support 200 isconstructed from may be selected to be resistant to frictional wear. Inone embodiment of an implant according to the invention, axial support200 is preferably constructed of titanium or a titanium alloy.

Where axial support 200 is a single piece of material it may befabricated from a polymer or composite of a polymer and otherreinforcement. Typical reinforcements include but are not limited tocarbon or glass fibers, either continuous or chopped in form. Otherreinforcements may be thin sheets of metals that are laminated togetherusing polymers as adhesives. The size, shape, orientation and amount ofthese reinforcements may be such that the mechanical properties of axialsupport 200 can be engineered to meet the flexibility and strengthrequirements.

It is also to be appreciated that torsional supports 300A, 300B may beconstructed of any material and in any manner that provides sufficientresistance to bending and torsion of implant 10 to allow implant 10 toprovide the torsional support function of the joint replaced, but thatis sufficiently flexible to allow implant 10 to bend or turn wheredesired, such as in twisting or bending of a spinal implant according tothe normal movement of the spinal column. Torsional supports may also beconstructed of a material and in a manner that is safe for a recipientof implant 10. Torsional supports 300A, 300B also may be constructed ina manner that provides sufficient resistance to bending and torsion toallow implant 10 to support surrounding tissue and prevent injury due toexcessive bending or torsion, For example, axial support 200 may beflexible and may not provide sufficient resistance to bending ortorsion. Accordingly, if torsional supports 300A, 300B do not providesufficient resistance to bending and rotation, implant 10 may allowover-rotation or excessive bending of a joint, potentially resulting ininjury. For example, where implant 10 is used to replace a spinal disc,over rotation or excessive bending could result in pain or damage to thenerves of the spinal column. Accordingly, torsional supports 300A, 300Bpreferably provide some resistance to bending and may also allowtorsional supports 300A, 300B to perform some of the shock absorbingfunction that the replaced joint had.

While torsional support 300A, 300B may provide some resistance totorsion or bending, torsional supports 30A, 300B are preferably providedso that this resistance should is not so great that desired motion ofthe joint is lost. For example, it may be desired to restore a fullrange of motion to a joint replaced by implant 10, and torsional support300A, 300B may have a degree of resistance to torsion and bending thatlimits the implant to the range of motion of the original joint, but notmore than this.

In one embodiment of an implant of the invention, torsional support300A, 300B may be comprised of one or more struts to provide resistanceto torsion and bending while still allowing desired motion. For example,one or more struts may extend from first plate 100 to second plate 102.The struts may be arranged such that, unlike axial support 200, pressuredirectly against plates 100, 102 at the ends of the struts will not beresisted by the struts due to their incompressibility, but rather, thestruts act as a spring. For example, the struts may be curved, or thetop and bottom of these struts may not be directly above one another. Asillustrated in FIGS. 1-4, struts may curve around some portion of axialsupport 200 while extending between plates 100, 102, providingsimultaneous flexibility and resistance both to torsion and to bendingof implant 10 around axial support 200.

It is to be appreciated where even resistance to bending and torsion isdesired for implant 10, torsional supports 300A, 300B may besymmetrical. For example, where there are two torsional supports 300A,300B, the torsional supports may be mirror images of one another asillustrated in FIG. 2, or where there are more than two torsionalsupports 300A, 300B, they may be equally spaced around axial support200. Where even resistance to bending and torsion is not desired,torsional supports 300A, 300B may be asymmetrical to provide moreresistance where more resistance is desired, or more torsional supportsmay be used at these locations.

It is to be appreciated that torsional supports 300A, 300B may beconstructed by any method that will provide desired properties and longlife. For example, torsional supports may be cast, machined or otherwiseformed and then attached to plates 100, 102, such as by welding.However, one possible disadvantage of manufacturing torsional supports300A, 300B separately from plates 100, 102, in other words other than asa unitary structure, is that the points of attachment may weaken and besubject to fatigue and possibly failure. Furthermore, if the struts arebent or twisted once formed, this deformation may result inmicro-cracking and other structural degradation. Accordingly, in oneembodiment of an implant according to the invention, it is preferredthat torsional supports 300A, 300B are integrally formed with plates100, 102, For example, plates 100, 102 and torsional support 300A, 300Bmay be formed by the LENS process. For example, implant 10 formed by theLENS process may be comprised of solid metal and may be formed in theexact shape desired, eliminating the need to attach torsional support300A, 300B to plates 100, 102 or to twist or bend torsional support300A, 300B.

It is to be appreciated that torsional supports 300A, 300B may beconstructed of any material that is safe for a recipient of implant 10,that can withstand the stresses that will be placed upon it, and thatalso has sufficient flexibility to allow desired motion of implant 10.For example, torsional supports 300A, 300B may be constructed of amaterial that is biocompatible. Torsional support 300A, 300B may also beconstructed of a material that is strong and durable enough to withstandthe forces that may be placed upon it once installed in an implantrecipient. For example, torsional supports 300A, 300B may be constructedfrom a material such that torsional supports 300A, 300B may not failunder stress or repeated bending normally experienced by a joint itreplaces. The ability of torsional supports 300A, 300B to withstandstresses also may be dependant on the shape and size of torsionalsupports 300A, 300B as well as their material of construction and, thus,size and shape of torsional supports 300A, 300B may also be consideredwhen selecting a material. In one embodiment of an implant of theinvention, torsional supports 300A, 300B are preferably formed of ametal, and this metal is the same as that used to form plates 100, 102to facilitate construction by the LENS technique. Accordingly, it isalso preferred that torsional supports 300A, 300B are constructed oftitanium or a titanium alloy.

The implant of the present invention will be further illustrated by thefollowing example which is intended to be illustrative in nature and notconsidered as limiting to the scope of the invention.

EXAMPLE

One suitable construction of an implant having a shape and designsubstantially in accordance with the present invention is provided bythe following combination of elements.

An implant 10 to be used in a spinal arthroplasty includes a first plate100 and a second plate 102. Plates 100, 102 are substantially oval andplanar and are sized to fit within a human spinal column in a spacepreviously occupied by a disc. The outer planar surfaces of plates 100,102 are provided with protrusions 400 consisting of teeth and a tissueingrowth region 500 consisting of a textured surface.

Implant 10 also includes an axial support 200, between, and connecting,plates 100, 102. Axial support 200 is oriented in the center of plates100, 102 and includes a cable incorporated at both ends to plates 100,102. Implant 10 further includes two torsional supports 300A, 300B.Torsional supports 300A, 300B are integrally formed with plates 100, 102and curve around axial support 200 such that the first end of each oftorsional supports 300A, 300B is not directly across from the second endof each of torsional supports 300A, 300B. Torsional supports 300A, 300Bare mirror images of one another. Implant 10 is constructed of an alloyof titanium with 6% aluminum and 4% vanadium by weight.

Implant 10 may has an outer envelope of approximately 20 mm in theanterior/posterior direction, 30 mm in the lateral direction, and 12 to15 mm in height. The overall shape mimics that of a vertebral body andis roughly kidney shaped. The size of torsional supports 300A, 300B aredependant on the material selected, but will be about 5 mm or less indiameter. Axial support 200 will be about 10 mm in diameter.

Having thus described at least one preferred embodiment of the implantand method of the invention, various alterations, modifications andimprovements will readily occur to those skilled in the art. Suchalterations, modifications and improvements are intended to be part ofthe disclosure and to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only and islimited only as defined in the following claims and equivalents thereto.

What is claimed is:
 1. An orthopedic implant, comprising: a first plate;a second plate; an axial support between the first plate and the secondplate and including one or more flexible struts; and one or moretorsional supports connected to the first plate and the second plate. 2.The orthopedic implant of claim 1, wherein the torsional supports eachcomprise a strut connected at a first end to the first plate and at asecond end to the second plate.
 3. The orthopedic implant of claim 2,wherein the first plate, the second plate and the one or more torsionalsupports each comprise a single, solid piece.
 4. The orthopedic implantof claim 3, wherein the struts are curved around the axial support. 5.The orthopedic implant of claim 4, wherein there are two struts and thestruts are mirror images of one another.
 6. The orthopedic implant ofclaim 3, wherein the torsional supports and the first and second platesare constructed of the same material.
 7. The orthopedic implant if claim6, wherein the material is biocompatible.
 8. The orthopedic implant ifclaim 7, wherein the material comprises titanium.
 9. The orthopedicimplant of claim 1, wherein the torsional supports, the first plate andthe second plate are integrally formed.
 10. The orthopedic implant ofclaim 9, wherein the struts are curved around the axial support.
 11. Theorthopedic implant of claim 10, wherein there are two struts and thestruts are mirror images of one another.
 12. The orthopedic implant ofclaim 9, wherein the torsional supports and the first and second platesare constructed of the same material.
 13. The orthopedic implant ifclaim 12, wherein the material is biocompatible.
 14. The orthopedicimplant of claim 13, wherein the material comprises titanium.
 15. Theorthopedic implant of claim 1, wherein in a first position the flexiblestruts are substantially perpendicular to the first plate and the secondplate and the first plate and the second plate are substantiallyparallel to one another.
 16. The orthopedic implant of claim 15, whereinthe flexible struts are cables.
 17. The orthopedic implant of claim 16,wherein the flexible struts are secured at a first end to the firstplate and at a second end to the second plate.
 18. The orthopedicimplant of claim 17, wherein the struts are secured by welds to thefirst plate and the second plate.
 19. The orthopedic implant of claim 1,wherein the axial support comprises a first half connected to the firstplate and a second half connected to the second plate.
 20. Theorthopedic implant of claim 19, wherein the first half mates with thesecond half.
 21. The orthopedic implant of claim 20, wherein the firsthalf is integrally formed with the first plate and the second half isintegrally formed with the second plate.
 22. The orthopedic implant ofclaim 1, wherein the first plate and second plate are substantiallyplanar.
 23. The orthopedic implant of claim 1, wherein the first plateand the second plate comprise one or more protrusions for attachment toone or more bones.
 24. The orthopedic implant of claim 1, wherein thefirst plate and the second plate comprise openings for inserting afixation device.
 25. The orthopedic implant of claim 1, wherein thefirst plate and the second plate each comprise a tissue ingrowth region.26. The orthopedic implant of claim 25, wherein the tissue ingrowthregion is porous.
 27. The orthopedic implant of claim 26, wherein thetissue ingrowth region comprises sintered metal particles.
 28. Anorthopedic implant, comprising: a first plate; a second plate; an axialsupport between the first plate and the second plate, having a firsthalf connected to the first plate that includes a ball and a second halfconnected to the second plate that includes a socket, the ball andsocket sized and arranged such that the first half and second half mate;and one or more torsional supports connected to the first plate and thesecond plate.